The present invention relates to a differential coding system and, more particularly, to a differential coding system applicable to a staggered quadrature amplitude modulation transmission system and an apparatus therefor.
Generally, a differential coding system regards a transmission line code as a kind of state and decides a state at the next time instant based on a current state and a current original data code, employing the resultant data sequence as a transmission code sequence. This method has been extensively used for the purpose of faithfully regenerating an original code sequence at the receiver side, even if state ambiguity exists in the transmission line, by observing a sftate transition only.
Typical examples of state ambiguity in the transmission system are the polarity ambiguity in the one-dimensional signal and the quadrant ambiguity in the two-dimensional signal. Differential coding effectively applicable to such simple ambiguities may be found by a method which is intuitively obvious. However, concerning a staggered quadrature amplitude modulation (SQAM) system which involves combined ambiguity of time ambiguity and phase ambiguity, it has been considered difficult to adopt differential coding therefor. For details of SQAM, a reference may be made to the paper entitled "Performance of Staggered Quadrature Amplitude Modulation in the Presence of Phase Jitter", IEEE Transactions on Communicaions, Vol. COM-23, No. 3, March, 1975.
As has been pointed out, SQAM signals have various advantageous features such as immunity to jitter and small peak power. It has also been pointed out that a so-called QAM transmission system which is strong to transmission distortion and impuse noise and high in frequency utilization efficiency is obtainable by orthogonally multiplexing a plurality of SQAM signals by use of a plurality carriers having different frequencies. Despite those outstanding advantages of SQAM, the number of transmission apparatuses heretofore realized with SQAM is distressingly small. Such a situation is due mainly to the fact that differential encoding has generally been believed impracticable when it comes to SQAM, as previously stated.